First and foremost, dialogue unveils characters. A playwright needs to have a good idea who his main players are and who they will become at the end of the play, inside and out. Characters in a play speak through their own vocabulary, accent, and life experiences.
Open the playscript of the famous play, Nora by Ibsen. See how in the beginning of the play, the husband plays the upper hand and Nora answers meekly. Then, observe how, at the end of the play, the dialogue has changed with the evolution of the plot and the characters.
As you are making your characters speak, make sure they are using their own words and not saying a pretty or clever idea the writer has put in their mouths. Even in plays written to underline some serious perspective or undertaking, like protesting social injustice, a player's dialogue should not consist of long tirades with excess verbiage.
To illustrate this point, let's look at an example. If you have a farmer Uncle John who is about to lose his farm and is only educated in the ways of the farm, he would not give a big speech with long words underlying the farm economy. Uncle John would not say in one breath, "Forming an equitable system of high-quality government is an imperative, for a good government must formulate farming to become profitable for whoever is disposed to be trained and is eager to exert bodily effort. The plight of the poor farmers is obvious or else most of the population will starve to death, and this capitalism will not save us from the farmer's